Known capacitor elements are obtained by subjecting an anode body composed of a sintered compact of tantalum powder or niobium powder to electrolytic oxidation in an aqueous solution of an electrolyte such as phosphoric acid to chemically convert the surface layer of the sintered compact into a dielectric composed of metal oxide. Further, various techniques for improving tantalum capacitor elements have been proposed (Patent Document 2).
Tungsten is known as one of valve action metals. When an anode body composed of a sintered compact of tungsten powder is subjected to electrolytic oxidation under the same conditions as the conditions of electrolytic oxidation of an anode body composed of a sintered compact of tantalum powder or niobium powder, tungsten oxide is crystallized in the formation process of a dielectric layer, thereby producing clods 13 with a flat surface, as shown in FIG. 2. This dielectric layer is not a sufficiently dense film with a suitable thickness, and seems to have many defects that electric current may flow when voltage is applied. Accordingly, conventional tungsten capacitors have large leakage current. Moreover, as shown in FIG. 2, narrow pores 12 make it difficult to uniformly form a semiconductor layer deep in the pores, which may reduce the capacitance. As a result, the obtained tungsten capacitor has a much lower capacitance in a high frequency range than in a low frequency range. Furthermore, in order to reduce leakage current, use of an alloy composed of tungsten and other metal is examined (Patent Document 1); however, the effect of reducing leakage current is slight.